1. Field of the Invention
This invention relates to an avalanche photodiode having a satisfactory guard ring effect and a high speed response, and a method of making the same.
2. Description of the Related Art
Nowadays, III-V avalanche photodiodes (abbreviated as APD) have been widely used as semiconductor light receiving devices for high speed optical communication in a long distance. The photodiode is responsive to light in a wavelength (1 to 1.6 .mu.m) in which a transmission loss of an optical fiber is low.
In the III-V avalanche photodiode (APD), a dark current flowing through an InGaAs layer is abruptly increased by a tunnelling current, when a high electric field is applied thereto. Usually, to avoid this, an InP layer having a wide band gap, in which a tunnelling current is rarely generated, is used as an avalanche region, and the InGaAs layer of a low carrier concentration (low impurity concentration) is used, so that a high electric field may not be applied to the InGaAs layer [SAM(Separated Absorption and Multiplication)-APD].
FIG. 6 shows a conventional SAM-APD, in which an n-type InP buffer layer 2, an n.sup.- -type InGaAs light absorption layer 3, an n-type InGaAsP intermediate layer 4, an n.sup.+ -type avalanche InP multiplication layer 5, and an n-type InP window layer 6 are epitaxially grown in sequence on an n+-type InP substrate 1. A p.sup.+ -type diffusion layer 7 and a p-type guard ring 8 are formed in the window layer 6 by selective diffusion or ion implantation techniques. An anti-reflection film 9 and a P-side electrode 10 are provided on top of the upper surface of the substrate 1, and an N-side electrode 11 is formed on the lower surface of the substrate 1.
In the SAM-APD thus formed, carriers generated by light absorption in the n--type InGaAs layer 3 are moved by drift to the n+-type InP layer 5 to perform the avalanche multiplication. Therefore, the tunnel current is suppressed, whereby the APD with the low dark current is obtained. However, since a heterojunction is formed between the InGaAs layer 3 and the InP layer 5, holes generated by the light absorption are accumulated in the barrier of a valence band existing on the heterojunction, resulting in a low response. To overcome the disadvantage, the intermediate layer 4 is interposed between the n.sup.- -type InGaAs layer 3 and the n.sup.+ -type InP layer 5, thereby reducing the barrier of the valence band. The composition of the intermediate layer 4 is given to have an intermediate energy gap between those of the n.sup.- -type InGaAs layer 3 and the n.sup.+ -type InP layer 5.
For realizing an APD of a low dark current and a high avalanche multiplication factor, it is necessary that uniform avalanche multiplication is performed over the entire light receiving surface, and that no voltage breakdown occurs in the region other than the light receiving region. Since the electric field tends to concentrate particularly in a curved portion 12 of a PN junction 15 between the n-type InP layer 6 and the p.sup.+ -type InP layer 7, the voltage breakdown may occur locally (called edge breakdown). To prevent the local voltage breakdown, an APD has been proposed which includes a guard ring 8 at the peripheral portion of the PN junction 15.
In general, the edge breakdown does not occur more easily in a graded junction, in which the impurity concentration of a semiconductor region near the PN junction varies linearly, than in an abrupt step junction in which the impurity concentration varies in a stepping manner. Therefore, usually, the PN junction in the light receiving portion is provided by the abrupt step junction, and that in the guard ring portion is given by the graded junction.
In the conventional APD as described above, the n-type InP window layer 6 of the low carrier concentration is epitaxially grown on the n.sup.+ -type InP avalanche multiplication layer 5 of the high carrier concentration. The PN junction between the layer 6 and the guard ring 8 is formed by implanting Be ions or the like into the layer 6 and annealing the implanted region at a high temperature. The PN junction 15 in the light receiving portion is formed by selective diffusion techniques, using Cd.sub.3 P.sub.2 as a diffusion source.
In order to obtain the guard ring effect in the conventional APD, it is necessary that the PN junction of the light receiving portion be formed at a shallow position of the n-type InP window layer 6, and the PN junction of the guard ring 8 be formed at a position deeper than the curved portion of the PN junction of the light receiving portion.
On the other hand, in order to obtain a high response, it is necessary that the carrier concentration of the InP avalanche multiplication layer 5 is increased to reduce the avalanche build-up time, and the electric field applied to the heterojunctions 13 and 14 is increased to prevent the storage of holes. For this reason, the PN junction of the light receiving portion is formed in a portion which enables the guard ring effect to be obtained, and as deep as possible near the n.sup.+ -type InP avalanche multiplication layer 5.
The thickness of the n.sup.+ -type InP avalanche multiplication layer 5 is determined in accordance with the controllability and uniformity of the crystal growth, and the depth of the PN junction 1 formed in the n-type InP window layer 6 is determined by the controllability of the diffusion. Therefore, for providing an APD having a satisfactory guard ring effect and a high speed response, controllability and reproducibility of the thickness of the layer 5 and the depth of the PN junction 15 become important. At any rate, the manufacturing process is complicated and the manufacturing yield is apt to become low.
As described above, in the conventional APD, in order to obtain the satisfactory guard ring effect, the depth of the guard ring must be greater than that of the flat portion of the PN junction in the n-type InP window layer. Moreover, for improving the high speed response, the PN junction in the n-type InP window layer must be formed as deep as possible near the n.sup.+ -type InP avalanche multiplication layer. The depth of the PN junction and the thickness of the n.sup.+ -InP avalanche multiplication layer adapted to the above conditions cannot be controlled or reproduced satisfactorily, resulting in low response and manufacturing yield.